The present invention relates to a method for using a directed beam of energy to selectively sinter metal powder to produce a part. Specifically, the invention relates to the use of selective laser sintering (SLS) in order to produce a full density metal part.
Solid Freeform Fabrication (SFF) is a group of emerging technologies that have revolutionized product development and manufacturing. The common feature shared by these technologies is the ability to produce freeform, complex geometry components directly from a computer generated model. SFF processes rely on the concept of layerwise material addition in selected regions. A computer generated model serves as the basis for making a replica. The model is mathematically sliced and each slice is recreated in the material of choice to build a complete object. A typical SFF machine is a xe2x80x9cminiature manufacturing plantxe2x80x9d representing the convergence of mechanical, chemical, electrical, materials and computer engineering sciences.
The first application of SFF technologies was in the area of Rapid Prototyping (RP). Rapid Prototyping enables design and manufacturing engineers to quickly fabricate prototypes in a fraction of the times and at typically less than half the costs in comparison with conventional prototyping methods. The tremendous economy of RP is facilitated by its high degree of automation, both in the design and fabrication cycles. On the design side, the advantages are at least fourfold. First, the use of Computer Aided Design (CAD) solid modeling software allows the design of a component to be stored digitally, obviating the need for detailed technical drawings and the extensive manual labor associated therewith. Second, the advent of so-called xe2x80x9cparametricxe2x80x9d CAD modeling software allows facile incorporation of design changes into an existing CAD design quickly. Third, the same CAD solid model can be fed to a variety of dynamic, mechanical and thermal analyses software, resulting in a high degree of design integration. Finally, each CAD model can be electronically xe2x80x9ctaggedxe2x80x9d for incorporation into databases that store information on parts assemblies, design variants and manufacturing methods. Lately, a move towards standardizing such information integration is taking place in the specification of the Standard for Exchange of Product Data (STEP). On the manufacturing side, computer driven RP machines accept the CAD solid model as input to automatically create a physical realization of the desired component. The major advantage realized here is the substantial elimination of process planning, operator expertise, additional tooling and set-up. The overall advantage of this powerful combination is the ability to rapidly iterate through several design and prototyping cycles before xe2x80x9cfreezingxe2x80x9d the design for final production at significantly lowered costs and shorter xe2x80x9ctime to marketxe2x80x9d.
Most RP technologies were initially developed for polymeric materials. These technologies allowed designers to rapidly create solid representations of their designs in a surrogate material for design visualization and verification. Further demand for functional prototypes led to the development of materials and processes that enabled production of prototype patterns and parts that could be subjected to limited testing for form and fit. Major developments have taken place next in the area of SFF known as Rapid Tooling. The focus of this area has been to develop SFF technologies to enable rapid production of prototype tooling for a variety of manufacturing techniques including injection molding, electro-discharge machining and die casting. The growth in this area has been spurred by the economical advantages of making limited run prototype tooling via SFF as compared to conventional techniques.
Over the past ten years, there has been an explosion in the development and growth of SFF technologies. These technologies can be broadly categorized into three classes, namely transfer, indirect and direct SFF methods. Transfer methods are those methods that use a pattern or sacrificial intermediary to generate the desired component whereas xe2x80x9cindirectxe2x80x9d methods are those SFF methods that directly produce intermediate density parts that undergo post-processing such as conventional sintering and infiltration to attain full density. Direct methods are methods that directly produce fully dense or near fully dense complex shaped parts in the desired composition (e.g. metal, ceramic or cermet) by applying a geometry and property transformation to the material with minimal post-processing requirements. In the context of making metal components by SFF, a number of xe2x80x9ctransferxe2x80x9d and xe2x80x9cindirectxe2x80x9d methods are available.
Selective laser sintering (SLS) is a SFF technique that creates three-dimensional freeform objects directly from their CAD models. An object is created by selectively fusing thin layers of a powder with a scanning laser beam. Each scanned layer represents a cross section of the object""s mathematically sliced CAD model.
Direct Selective Laser Sintering (Direct SLS), the relevant field of this invention, is a direct SFF technique. Direct SLS is a laser based rapid manufacturing technology that enables production of functional, fully dense, metal and cermet components via the direct, layerwise consolidation of constituent powders. In Direct SLS, a high energy laser beam directly consolidates a metal or cermet powder to a high density ( greater than 80%), preferably with minimal or no post-processing requirements. The main advantages associated with this technique are elimination of expensive and time-consuming pre-processing and post-processing steps. In comparison with xe2x80x9cindirect SLSxe2x80x9d, direct SLS is a binderless process. Direct SLS also does not involve furnace de-binding and infiltration post-processing steps as in xe2x80x9cindirect SLSxe2x80x9d. Compared to conventional bulk metal forming processes (e.g. casting or forging), direct SLS does not require the use of patterns, tools or dies. The metal powder being processed by direct SLS directly undergoes a shape and property transformation into final product that may require minimal post-processing such as finish machining.
Several processing requirements differentiate direct SLS of metals from SLS of polymers or polymer coated powders. Perhaps the most important distinguishing characteristic is the regime of high temperatures involved in direct SLS of metals. At the temperatures necessary for processing metals of interest (generally  greater than 1000xc2x0 C.), the reactivity of the melt poses serious process control issues. Control of the processing atmosphere takes on paramount importance since it not only enables successful layerwise buildup but also addresses safety concerns. In one particular application of SLS known as SLS/HIP, the goal of in-situ containerization of a part fabricated during SLS processing requires that it take place under vacuum to ensure full densification of the canned shape during HIP post-processing.
Until recently, no work was reported on direct SLS of high performance materials, such as Nickel and Cobalt base superalloys, superalloy cermets, Titanium base alloys and monolithic high temperature metals such as Molybdenum. These materials are used for high performance components that typically experience high operating temperatures, high stresses and severe oxidizing or corrosive environments. Direct SLS, with its ability to produce components in such materials is especially useful for functional prototype, low volume or xe2x80x9cone of a kindxe2x80x9d production runs. To manufacture a typical prototype lot of 100 superalloy cermet abrasive turbine blade tips, direct SLS can achieve acceptable microstructure and properties with 80% cost savings over the traditional methods. Aerospace industries face typical lead times of several dozen weeks for functional, metallurgical quality prototypes. Direct SLS can lower cost and drastically reduce lead times by eliminating pre-processing and post-processing steps, and by eliminating the need for specialized tooling.
A new, hybrid net shape manufacturing technique known as Selective Laser Sintering/Hot Isostatic Pressing (SLS/HIP) exploits the freeform shaping capabilities of SLS combined with the full densification capability of HIP to rapidly produce complex shaped metal components. SLS/HIP is a significantly faster, low cost, highly automated, flexible replacement for conventional powder metallurgy and HIP processes that are currently employed in the naval and aerospace defense sectors for the manufacture of low volume or xe2x80x9cone of a kindxe2x80x9d high performance components.
The present invention includes a directed energy beam, such as a laser, and is adaptable to produce almost any single layer or multi-layer three dimensional metal part.
Broadly speaking, the method comprises sequentially depositing layers of metal powder into a chamber with a partial pressure atmosphere. Once a layer of powder is deposited, a scanning laser beam selectively fuses the layer of the powder into the desired shape. The process continues until a nonporous or fully dense part is completely fabricated, layer by layer.
In the preferred embodiment, the laser does not follow the traditional raster scanning path. Rather, the laser employs a continuous vector (xe2x80x9cCVxe2x80x9d) mode of scanning, which allows each individual motion segment to take place in an arbitrary direction, but treating successive segments as part of a continuous path. Typically, the scan follows the path of a parametric curve such as an Archimedes spiral or another arbitrary, piecewise parametric curve that follows the contours of the outer boundary of the scan, such that a constant melt pool is always maintained under the laser beam. In the preferred method, the scan path is further modified so that the scan begins at a point inside of the area to be fully densified, where a finite starting radius for the path is defined. The scan begins by tracing a circle with the starting radius for the path and repeats scanning around the starting radius up to 20 times. Simultaneously ramping the laser power to a terminal power while doing so creates a melt pool at the center while avoiding balling that takes place at the location of initial incidence of a laser beam on a metal powder bed. Further, in the preferred method, the laser beam is allowed to oscillate in a path perpendicular to the scan path of the laser, so as to stop any forward velocity of the melt that may break up the continuity of the solid-liquid interface.
In the preferred form, inert gas is allowed to enter the chamber by a controlled leak, so as to reduce vaporization of the metals or alloying elements under the beam, as well as to reduce condensation of any vaporized metal on the laser window or other relatively cold areas of the chamber. The leak should result in a vacuum level of 10xe2x88x922 to 10xe2x88x924 Torr, preferably a vacuum level of 10xe2x88x923 Torr.
Additionally, in another preferred embodiment of the present method, the speed and power of the laser beam is such that there is a constant melt pool under it. Further, another embodiment sets the scan spacing of the beam to achieve the same result.
In yet another embodiment, after a supporting layer is built by sintering, the next layer is skipped, leaving the supporting layer with an oxide surface. Such a surface acts as a well-defined stop-off layer, exploiting a condition normally avoided at great expense.
As can be appreciated from the above general description, the present invention solves many of the problems associated with known part production methods. By using the techniques described above, fully dense metal components can be formed by Direct SLS. These techniques are also useful for fabrication of integrally canned shapes for SLS/HIP processing. An integrally canned shape can be thought of as composed of four distinct regions: the bottom xe2x80x9cend-cap,xe2x80x9d the top xe2x80x9cend-cap,xe2x80x9d the skin, and the interior powder core. The end-caps and the skin make up the xe2x80x9ccan,xe2x80x9d or the container portion that must be nonporous and leak free. The interfaces between the end-caps and skin, as well as the interfaces between adjacent skin layers must be leak free, necessitating their full density fabrication. Such skins need not be removed by machining or chemical etching, thus avoiding the delays, expense, and environmental or safety concerns associated with such techniques.